Many bicycles include manually shifted drivetrains having one or more derailleurs and sprockets. The derailleurs cause a chain to move between the sprockets in the drivetrain. The derailleurs are manually shifted via shifting mechanisms mounted on the handlebars or other rider accessible locations on the bicycle. Thus, the rider has to initiate shifting input to alter the gear ratio in the bicycle. This type of manual shifting input can be difficult to master for both inexperienced and experienced riders. A rider may not actuate the manual shifting controls with the proper force, timing, or direction, and as a result the rider's intended shifting operation may not be timely or properly executed. As a result, in inefficient gear ratio may result, and the rider's pedaling efficiency may suffer.
Automatic drivetrains have been developed for bicycles to eliminate the need for a rider to shift, increasing pedaling efficiency and simplifying bicycle operation. However, prior automatic drivetrains may only take into account a bicycle's cadence when determining a selected speed stage (e.g., gear ratio) in the drivetrain. Other factors including various resistances imparted to the bicycle, such as rolling resistances, drivetrain resistances, gravitational force, etc., can also affect pedaling efficiency and shifting performance.